Oxidation catalyst



Patented Apr. 6, 1954 OXIDATION GATALYST Joseph R. Darby, WebsterGroves, Mo., assignor to Monsanto Chemical Company, St. Louis, Mo., acorporation of Delaware No Drawing. Original application July 1, 1949,

Serial No. 102,699,. no dated January 13, 195

w Patent No. 2,625,554,

Divided and this application January 2, 1953, Serial No. 329,451

(El. 252-- i56) 10 Claims.

ihis invention relates to. an improved process for the vapor phaseoxidation of benzene t maleic anhydride.

Various processes including the use of a large number of catalyticbodies have been suggested for use in the vapor phase oxidation oforganic materials including the oxidation of benzene to maleicanhydride. While effective results are obtained by employing many ofthese .processes and catalytic materials, there is still considerableroom for improvement;

It is an object of'this'invention to provide an improved process for thevapor phase catalytic oxidation of benzene to malelc anhydride. Aparticular object of this invention is to provide an improved catalyticbedfor the vapor phase oxidation of benzene.

According to the present invention, benzene is oxidized to maleicanhydride by contacting a mixture of benzene and oxygen with a bed ofcatalytic particles comprising a porous, inert carrier body composed offinelyvdivided silica, the particles of which are bonded together byfinely divided particles of a refractory glass which is low in basicconstituents, said carrier being impregnated with a catalytically activematerial containing vanadium and substantially all the particles in saidcatalyst bed being of such a size as to be retained on a 6 mesh screenand at least the last 50% of said bed contacted by the reactants beingmade up substantially of particles of such a size as to pass through a 4mesh screen. Thus it has been discovered that by employing a bed ofcatalytic particles ofthis size and composition, unexpectedly highyields of maleic air-hydride are obtained.

The composition of thecarrier body. employed according to this inventionis described and claimed in United States Patent 2,229,361 to Johann A.Bertsch. It comprises finely ground silica or material rich in silicamixed with a finely ground refractory glass of the Pyrex type, that is,one which is low in basic constituents such as sodium, potassium andcalcium, and then heated to a temperature .sufficient to soften theglass to obtain bodies suitable for catalyst carriers. Ihese carriersare mechanically very hard and strong, substantially chemically inertwhen used in the catalytic oxidation of organic materials, and ofsufficient porosity to absorb or otherwise hold and retain acatalyticmaterial in relatively permanent association.

A highly convenient method of preparing such carriers comprises addingto the-pulverized silica and glass a temporary and volatile binder'whichholds the material as a coherent mass-during the preliminary stages offorming but disappearsduring calcination.

The preparation of the catalyst carrier composition is susceptible ofnumerous modifications. However, the following constitutes -aspecificexample of one method'whichhas been found to be highly satisfactory.Silica-ora-highlysilicious material is ground or-otherwise' reduced toarelatively fine state, for example; tosuch state'as to admit of itspassage through ascreen of 200 mesh and is then admixed witlrasuitablepei'centage (c. g. 15%) of ground and relatively refractoryglass. A glass of-high silicacontent is: preferred. The glass should be"relatively" low in. basic constituents since these tend to promoteundesirable side reactions. The glass sold-under the commercial name ofPyrex, which consists: mainly of silica, an amount of boron togetherwith relatively small proportions of basic-materials is found to bequite satisfactory. Common window glass, for example, is undesirablesince it-contains large proportions of basic constituents. These basicmaterials function ascombustion catalysts and a catalyticmass-'containing'appreciable proportions of them show an unsatisfactoryconversion of the organic material to the desired product and largeamounts of carbon dioxide and Water are produced; The-finenessof'grinding of the glass may vary'toa-considerable extent but 100 meshmaterial is foundto give good results.

After mixing for purposes of forming this composition into pelletssuitable for calcination, it is desirable to add to the mixture ofsilica and ground glass a temporary binder, for example, an organicsubstance which, while it acts as abinder for the uncalcined material,is susceptible of volatilization without leaving an appreciable amountof residue in the final product. Anexcellent binder may be prepared byadmixing stearic acid with cyclohexanol, the amount of the first beingl2% by weight of the silica-pyrex'mixture and that of the second being15%; when admixed with the silica-glass mixture forms a dough which canbe easily molded into pellets or other convenient shapes.

An especially eiiective catalyst support is. obtained by shaping thedough into small cylinders whose axes and diameters are of equaldimensions. Supports of this form are not subject to packing and givelow resistance'to-the flow-of gas. After the pellets have been formedthey may be dried with air (preferably'a stream of air) at a temperatureof 100 Ci Wherrtheyare This binder sumciently dried to admit ofhandling, they are placed in an ordinary muflle furnace and burned atbright red heat for three to four hours. In this burning operationsubstantially all of the organic binder is evaporated or completelyburned and. there remain hard, refractry, porous, white, waterinsoluble, chemically inert pellets of very regular configuration.

The catalytic material may be applied to these pellets in any convenientmanner. One method involves forming a solution of the soluble salts ofcatalysts for example, a saturated solution of ammonium vanadate andthen applying the solution by spraying or dipping. The solution isabsorbed into the pellet to give a highly intimate and permanentassociation between catalyst and carrier. A second method, which may beconveniently employed, involves merely dusting the pellets with finelyground catalytic material. A third method involves fusing the catalystand then dipping the carrier pellets thereinto. In this last method itis found that the pellets act as sponges to take up the molten catalyticsubstances to give intimate and permanent association. If the catalystis applied as a powder, it may be caused to enter into the pores of thecarrier and thus to be intimately bonded thereto, by subjecting thepellets to a second step of calcination at a temperature sufiicient tofuse the catalyst to the carrier. The intimacy of the bond betweencarrier and catalyst as obtained by spraying or dipping the carrier withsolutions of the catalytic substances also may be intensified by heatingthe coated pellets to the fusion point of the deposited catalyticcompound.

Components of glass may also be admixed with large amounts of silica,and this mixture upon heating will give a partially fused mass useful asa catalyst carrier.

In forming the catalyst it is desirable to leach the carrier pellet witha dilute acid solution, for example, hydrochloric acid, beforeimpregnation with the catalytic material. According to one of thepreferred embodiments of the invention, the carrier is leached with anacid solution, for example, a dilute, e. g. 5-10% aqueous solution ofsuch acids as hydrochloric acid, sulfuric acid, maleic acid and otherstrong organic and inorganic acids having at least one hydrogen with adissociation constant of at least 1X10"? The leaching is preferablycarried out at an elevated temperature, e. g. 80-100" C., and continueduntil a fresh leaching solution develops little or no color. Thereafterthe leached carrier is thoroughly washed with water, the last traces ofacid neutralized by treating the carrier with a volatile base, e. g.immersing the carrier in an aqueous solution of such non-metallic basesas ammonia, ethanolamine, diethanolamine, triethanolamine,cyclohexylamine and other organic bases. Thereafter the carrier isthoroughly washed with water and dried.

The following examples are illustrative of the present invention but arenot to be construed as limitative thereof.

Example I Carrier pellets are prepared as described above in the form ofsmall cylinders having an axis of about 0.16 inch in length and adiameter of about 0.16 inch. These pellets are coated with partiallyreduced vanadium pentoxide catalyst. A vertical reaction tube one-halfinch in inside diameter and 80 inches long is filled with the coatedpellets and a mixture of air and benzene in an approximate ratio of 1pound of benzene to 15 pounds of air at a temperature of 450 C. ispassed through this tube from top to bottom. It is found that maleicanhydride is produced in a high yield.

In contrast to the high yields of the process described above in ExampleI, when the same process is carried out except that the coated pelletsused in Example I are replaced by coated pellets of similar shape buthaving axis and diameter dimensions of 0.22 inch instead of 0.16 inch,the yield of maleic anhydride is only 80% of that of Example I.

In further contrast to the process of the invention as exemplified byExample I, when the catalytic particles comprise coated carrierparticles of smaller size, i. e., are too small to be retained by a 6mesh screen, various difliculties arise in carrying out the process, forexample there is a substantially higher pressure drop through the tubewhich adds to the expense and danger of the operation, the useful lifeof the catalyst is substantially shorter, etc.

As pointed out above, substantially all of the particles of catalyst inthe catalyst bed are of such a size as to be retained by a 6 mesh screenand at least the last 50% of the bed contacted by the reactants, i. e.the gas mixture comprising benzene and oxygen is made up substantiallyof particles of such a size as to pass through a 4 mesh screen.

While all of the catalyst bed may be made up entirely, or substantiallyentirely, of particles which pass through a 4 mesh screen but areretained by a 6 mesh screen, as pointed out above up to 50% of the bedinitially in contact with the reactants may be made up entirely, orsubstantially entirely, of larger particles, e. g. particles which passthrough a 1 mesh screen and are retained by a 4 mesh screen. A morepreferred range of larger particles are those which pass through a 3mesh screen and are retained by a 4 mesh screen. According to apreferred embodiment of the invention the first 10-50% of the bedcontacted by the reactants is made up substantially of such particles.

Thus, according to this embodiment of the invention, the portion of thecatalytic mass that is initially contacted by the mixture of benzene andoxygen may be made up of large particles, i. e., particles retained by a4 mesh screen, for example, cylindrically shaped particles having adiameter and an axis length of 0.22 inch. Thus, it is surprisinglydiscovered that even though the over-all length of the catalytic mass isnot increased, the improved yields attained by the use of smallercatalytic particles are retained, while at the same time the pressuredrop through the reaction tube is substantially lower.

The following example exemplifies the use of a catalytic mass in whichthe mixture of benzene and oxygen comes in contact initially with largercatalytic particles.

- Example [I Example I is repeated except that the top half of thereaction tube is filled with coated carrier particles in which the axisand diameter dimensions are 0.22 inch instead of 0.16 inch. A similaryield of maleic anhydride is obtained while at the same time thepressure drop through the tube after 1000 hours of operation is only 2.5pounds per square inch in contrast to 5 pounds per square inch inExample I. It is further discovered that the catalytic mass of theexample has an even longer useful life than the catalytic mass employedin Example I.

The process of the invention is carried out by passing mixturescontaining oxygen and benzene,

for example, mixtures of air and benzene over catalytic particlescomprising a vanadium catalyst dispersed on carrier particles having thecomposition and size characteristics set forth above. The catalytic massor bed may be contained in any suitable vessel and such vessels may havevarying shapes. However, for most purposes, it is generally preferredthat the catalytic bed be contained in an elongated vessel as forexample, a metallic tube such as a mild steel tube. These tubes may varysubstantially in diameter and length depending upon the magnitude of theoperations, for example from V inch to 3 inches or more in diameter andfrom 1 foot to 10 feet or more in length. It is to be understood,however, that the process of the invention is not limited to anyparticular size or shape of the catalytic bed.

The catalyst which may be employed according to the invention is avanadium type catalyst, i. e., a vanadium oxide, e. g., vanadiumpentoxide, partially reduced vanadium pentoxide, etc., or salt or othercompound thereof and mixtures of such vanadium compounds and othermetallic compounds, examples of which are known to those skilled in theart. Included among such auxiliary metal compounds are oxides and saltsof aluminum, titanium, iron, cobalt, zinc, copper, nickel, molybdenum,magnesium, manganese, chromium etc. Usually the catalyst includes only aminor amount of auxiliary metal compound, for example, 2-20% based onthe total amount of the catalyst and an even more preferred proportionis 15%.

In carrying out the oxidation of benzene, it is usually preferred thatthe oxygen be diluted by an inert gas and for this reason air is usuallyemployed as the oxidizing medium. The ratio of air to benzene may besubstantially varied but is usually within the range 6 to 30 pounds ofair for each pound of benzene.

The oxidation process is carried out at elevated temperatures andusually temperatures in the range 3G0-600 C. are used, a preferredtemperature range being 425-500" C.

The mesh sizes referred to herein are those of the Tyler Standard ScreenScale and are equivalent to the United States series. Thus, the 3 meshsize has an opening of 0.263 inch, the 4 mesh size has an opening of0.185 inch and the 6 mesh size has an opening of 0.131 inch.

The catalytic particles of the invention may be made up in variousconfigurations provided at least one dimension thereof is within therange of mesh sizes set forth above. Preferably, however, the particlesare more or less uniform in shape, i. e. they more or less approximatespheres, cubes, cylinders and the like. As pointed out above aparticularly useful shape is that obtained by forming the carriercomposition into cylinders having axes and diameters of approximatelyequal dimensions. Catalyst beds made up of particles of varyingconfigurations may also be used.

Preferably all of the particles in the catalyst bed are of the sizegiven hereinbefore but the improved results of the invention areessentially attained even though a few percent e. g. l10% by weight ofthe particles are outside of the above limits, particularly if suchparticles are relatively uniformly distributed through the bed.

This application is a division of my copending application Serial Number102,699, filed July 1, 1949, now Patent No. 2,625,554.

What is claimed is:

1. A catalyst for the vapor phase oxidation of benzene to maleicanhydride which comprises a porous, inert mass of finely subdividedsilica, the particles of which are bonded together by a matrix of arefractory glass which is high in silica and low in basic constituents,the bonded silica particles being impregnated with a catalyticallyactive material containing vanadium, the portion of the catalystic bedwhich is to be in initial contact with the mixture of benzene and oxygenbeing made up substantially of particles of such a size as to beretained by a 4 mesh screen and the balance of the bed being made upsubstantially of particles of such a size as to pass through a 4 meshscreen and be retained by a 6 mesh screen.

2. A catalyst as defined in claim 1 in which the catalytic particles arecylindrical in shape and have axes and diameters of substantially equaldimensions.

3. A catalyst for the vapor phase oxidation of benzene to maleicanhydride which comprises a. bed of particles comprising a porous, inertcarrier body composed of finely divided silica, the particles of whichare bonded together by finely divided particles of a refractory glasswhich is low in basic constituents, said carrier being impregnated witha catalytically active material containing vanadium and substantiallyall of the particles of said bed being of such a size as to be retainedby a 6 mesh screen and at least the last of said bed to be contacted bythe reactants being made up substantially of particles of such a size asto pass through a 4 mesh screen and be retained by a 3 mesh screen.

4. A catalyst as defined in claim 3 in which substantially all of theparticles are substantially uniform in shape.

5. A catalyst as defined in claim 4 in which the carrier particles arecylindrical in shape and have axes and diameters of substantially equaldimensions.

6. A catalyst as defined in claim 5 in which the catalytically activematerial comprises partially reduced vanadium oxide.

'7. A catalyst as defined in claim 3 in which the catalytically activematerial comprises partially reduced vanadium oxide.

8. A catalyst as defined in claim '7 in which the initial 10-50% of thecatalyst bed is made up substantially of the larger particles and thebalance substantially of the smaller particles.

9. A catalyst as defined in claim 1 in which the catalytically activematerial comprises partially reduced vanadium oxide.

10. A catalyst as defined in claim 1 in which the initial 10-50% oi thecatalytic bed is made up substantially cf the larger particles and thebalance substantially of the smaller particles.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,510,803 Cooper June 6, 1950 2,625,554 Darby June 13, 1953

1. A CATALYST FOR THE VAPOR PHASE OXIDATION OF BENZENE TO MALEICANHYDRIDE WHICH COMPRISES A POROUS, INERT MASS OF FINELY SUBDIVIDEDSILICA, THE PARTICLES OF WHICH ARE BONDED TOGETHER BY A MATRIX OF AREFRACTORY GLASS WHICH IS HIGH IN SILICA AND LOW IN BASIC CONSTITUENTS,THE BONDED SILICA PARTICLES BEING IMPREGNATED WITH A CATALYTICALLYACTIVE MATERIAL CONTAINING VANADIUM, THE PORTION OF THE CATALYSTIC BEDWHICH IS TO BE IN INITIAL CONTACT WITH THE MIXTURE OF BENZENE AND OXYGENBEING MADE UP SUBSTANTIALLY OF PARTICLES OF SUCH A SIZE AS TO BERETAINED BY A 4 MESH SCREEN AND THE BALANCE OF THE BED BEING MADE UPSUBSTANTIALLY OF PARTICLES OF SUCH A SIZE AS TO PASS THROUGH A 4 MESHSCREEN AND BE RETAINED BY A 6 MESH SCREEN.